Ah! I see. The D-type changes state (toggles) on the low to high (0 to 1) transition of the switch.

This is a divide-by-two circuit. Used in frequency dividers and binary counters.

Bit more explanation:-
On the low to high transition of the clock input, the state of the D input gets transferred to the Q output.
Because the /Q output (the opposite of Q) is connected to the D input, on every clock the Q output changes state.